Lecture no. 1

Principles 

          Electromagnetic energy refers to all energy that moves with the velocity of light in a harmonic wave pattern. The wave concept explains the propagation of electromagnetic energy, but this energy is detectable only in terms of its interaction with matter. Electromagnetic radiation consists of an electrical field (E) which varies in magnitude in a direction perpendicular to the direction in which the radiation is traveling, and a magnetic field (M) oriented at right angles to the electrical field. Both these fields travel at the speed of light (C). A number of interactions are possible when electromagnetic energy encounters matter depending on its properties, whether solid, liquid or gas. Energy may be (i) transmitted, through the substance, (ii) absorbed by a substance, (iii) emitted by a substance, (iv) scattered, i.e., deflected in all directions and lost, and ultimately (v) reflected. If it is returned unchanged from the surface of a substance with the angle equal and opposite to the angle of incidence, it is termed specular reflectance (as in a mirror). If radiation is reflected equally in all directions, it is termed diffuse. Real materials lie somewhere in between. The science of remote sensing detects and records changes in electromagnetic radiation by magnitude, direction, wavelength, polarization and phase. The resulting images and data are interpreted remotely to identify the characteristics of the matter that produced the changes in the recorded electromagnetic radiation. 

Basic Processes of Remote Sensing 

          Basic processes of remote sensing are depicted in pictorial form and are listed as follows: 

·         Energy source (sun or transmitter) 

·        Transmission of energy from source to object 

·         Energy interaction with object surface 

·        Transmission of energ y to sensor 

·        Scattering and absorption by atmosphere 

·         Detection, measurement and output by sensor 

·         Data acquisition, recording, pre-processing and an

alysis/interpretation 

Types of Remote Sensing 

Based on Source of Energy 

Passive Remote Sensing: It makes use of sensors that detect the naturally reflected or emitted electromagnetic radiation and are called passive sensors.

Active Remote Sensing: It supplies its own source of energy which is directed at the object in order to detect reflected responses from objects, such as radar, airborne laser and are known as active sensors. 

Based on Spectral Regions of Electromagnetic Radiation used 

Visible and Reflective Infrared Remote Sensing: The energy source used in the visible and reflective infrared remote sensing is the sun. The sun radiates electro-magnetic energy with a peak wavelength of 0.5 µm. Remote sensing data obtained in the visible and reflective infrared regions mainly depends on the reflectance of objects on the ground surface. Therefore, information about objects can be obtained from the spectral reflectance. Spectral range covered is 0.4 to 3 µm. 

Thermal Infrared Remote Sensing: The source of radiant energy used in thermal infrared remote sensing is the object itself, because any object with a normal temperature (300 °K) will emit electro-magnetic radiation with a peak at about 10 µm.

Therefore, in the wavelength region shorter than 3.0 µm, spectral reflectance is mainly observed, while in the region longer than 3.0 pm, thermal radiation is measured. Spectral range for thermal remote sensing is 3 to 14µm. 

Microwave Remote Sensing: There are two types of microwave remote sensing, passive microwave remote sensing and active microwave remote sensing. In case of the former, the microwave radiation emitted from an object is detected, while the back scattering coefficient is detected in case of the latter. Microwave remote sensing covers spectral range of 0.1 to 100cm.